The present invention generally relates to Surface Acoustic Wave (SAW) devices, and, more particularly, to the application of such devices to a unique word detector and coherent demodulator for a digital code differential phase-shift signal. Such a detector and demodulator has particular application in communications systems, and the use of the present invention results in an improvement in the unique word error rate (UWER), with a simpler structure.
It is common in TDMA communications systems to transmit a frame of data preceded by a preamble. The preamble typically includes a clock recovery sequence, a unique word and various housekeeping signals. A "unique word" is a digital word having a minimum of ten symbols and exhibiting high auto-correlation properties. Detection of the unique word is used for frame synchronization.
SAW devices have been used for performing numerous functions, such as bandpass filters, correlators, coders, decoders, modulators and the like, at radio frequencies between 10 M bit and 1 G bit. A unique word detector can be implemented in a straightforward manner with SAW technology. The fingers of the interdigital output transducer are arranged in a pattern corresponding to the unique word to be detected. In other words, the interdigital output transducer is phase-coded to have an impulse response equal to the time inverse of the unique word differential phase-shift signal waveform. A sharp correlation peak occurs when the unique word signal just fills the phase-coded output transducer. When implemented in this manner, the SAW device is a differential non-coherent decoding element--that is, it has 0.degree. and 180.degree. phase shifts at each finger point, depending on its location, without being able to identify the absolute phase. In other words, the SAW device, used as a unique word detector, will detect not only the unique word but its inverse as well without discriminating between the two. This is, of course, undesirable in a TDMA communication system because of data decoding errors resulting from failure to properly detect the unique word and establish frame synchronization. In addition, the inverse word is often used for "super-frame" synchronization.